Non-reciprocal phase shifter and attenuator



United States Patent Qfifice 3,048,831 Patented Aug. 7, 1962 3,048,801NON-RECIPROCAL PHASE SHIFTER AND ATTENUATOR Robert L. Fogel, Torrance,and Herbert T. Suyematsu,

Los Angeles, Calif., assignors to Hughes Aircraft Company, Culver City,Calif, a corporation of Delaware Filed June 8, 1959, Ser. No. 818,919 3Claims. (Cl. 333-31) The present invention relates to non-reciprocalwave transmission and, more particularly, to a coaxial slowwavestructure, partially disk-loaded, having selectively magnetized ferriteelements.

A number of non-reciprocal wave trans-mission devices have beendeveloped for microwave frequencies. The more recent, and most effectiveof these, utilize magnetized ferrite elements suitably mounted in awaveguiding structure to provide the required non-reciprocalcharacteristics. The principle of operation upon which these devices arebased is the action of the magnetized ferrite element to pass energy inone direction and attenuate or absorb the energy in the other direction.For this operation to attain, it is necessary that the ferrite elementbe mounted Within the waveguiding structure at a position Where is a netcomponent of circularly polarized radio frequency magnetic field of themode that is propagating.

In most of the known non-reciprocal ferrite devices, the circularlypolarized radio frequency magnetic field exists in the mode of theenergy that propagates in the absence of the ferrite element. Theferrite is placed in a position so that the resulting mode has itsmaximum component of circularly polarized magnetic field in the regionof the magnetized ferrite. Thus, for a rectangular waveguide propagatingenergy in the dominant mode, the ferrite element is mounted within thewaveguide to include a position located half way between thelongitudinal center line and one side of the broad walls. While thisstructure is the more common, it is possible under certain conditions toobtain non-reciprocal wave transmission by placing the ferrite elementin a position where the radio frequency magnetic field is linearlypolarized. Upon magnetization of a ferrite so positioned, a component ofcircularly polarized magnetic field is created within the ferritebecause of the gyromagnetic action. The non-reciprocal efiects resultingfrom this type of operation are second order, however, and lessefiicient than the previously described operation.

Non-reciprocal devices as described in the foregoing paragraphs aresuitable for operation at the higher microwave frequencies; however,when such devices are designed for operation at the lower microwavefrequencies and at the ultra-high frequency region, they are extremelylarge. Since coaxial transmission lines are smaller, they are moresuitable for the propagation of energy at such referenced lowerfrequencies and a new approach is required because the radio frequencymagnetic field of the dominant TEM mode is linearly polarized. Secondorder non-reciprocal effects may be readily achieved but for the moreeflicient first order non-reciprocal effects, one of several otherapproaches appear to be more feasible.

One of the referenced aproaches for achieving first order non-reciprocaleffects in coaxial transmission line involves the propagation of thefirst higher order T=E mode which has a circularly polarized componentof radio frequency magnetic field. In order to propagate this TE modewithin a given size coaxial line, the frequency required is much higherthan that of the TEM mode for the same line, or for a given frequencythe size of the line required to support the TE mode is much larger. Thedifferences are such that the advantages of the coaxial line over thehollow waveguiding structures are lost. Also energy progagated in thisTE mode tends to be converted into the dominant TEM mode by mechanicaland electrical asymmetries in the line, such as bends, junctions, etc.

Another approach for obtaining first order non-reciprocal efiects incoaxial transmission line is to provide the line with an inhomogeneouscross section so that the TEM mode cannot be progagated. The dominantmode for this modified type of coaxial line is a hybrid mode containingboth the TE and TM modes, although in a few special cases, the dominantmode can be solely the TE or the TM mode. This principle has beenutilized with coaxial lines in which the line has been half filled witha material having a high value of dielectric constant. The boundaryconditions imposed by this geometry require the existence oflongitudinal components of the radio frequency magnetic field which incombination with the circular components of the radio frequency magneticfield provides the required circular polarization at the air-dielectricinterface. With rods of ferrite material disposed along the interfaceand in presence of a static magnetic field non-reciprocal wavetransmission is achieved. Several geometries for the dielectric loadingof the coaxial line have been explored together with the suitablepositioning of the ferrite elements for non-reciprocal Wavetransmission.

In'this latter type of coaxial structure, i.e., a structure in which theinhomogeneity is non-circularly symmetric, the relation between themagnitude of the circularly polarized radio frequency magnetic field andthe characteristics of the dielectric loading material is not rigorouslyknown because the exact solutions to the wave equation have not yet beenobtained. Experimental and approximate theoretical results show that themagnitude of the non-reciprocal effects increases as the value of thedielectric constant for the loading material increases.

To more clearly understand the foregoing results in coaxial transmissionline having an inhomogeneous cross section, the difference between thefree-space velocities of propagation of the separate media will beassumed to cause a distortion of the principal mode. If the coaxial lineis completely filled with either of the different media, a TEM mode willpropagate with the free-space velocity of that medium and there will heno longitudinal component of the radio frequency magnetic field. If thecross section of the line is made inhomogeneous, the energy will try topropagate in each medium with the freespace velocity of propagation ofthat medium. Since the energy must propagate with the same velocity overthe entire cross section, however, the velocity of propagation of theresulting distorted mode (in the general case, a hybrid TE plus TM) willhave a value between the limiting values of the free-space velocities ofpropagation of the individual medium and the hybrid mode, in the caseunder consideration, will have a component of circularly polarized radiofrequency magnetic field. It is, therefore, readily apparent that as theratio of velocities of propagation of the two media is increased, theamount of distortion of the original mode (and, thus in the presentinstance, the magnitude of the component of the circularly polarizedradio frequency magnetic field) is also increased.

While non-reciprocal wave transmission is attainable in accordance withthe foregoing, it is to be recognized that in dielectric materials, thedielectric loss increases as the value of the dielectric constantincreases. As a result of such relationship, the dielectric lossinvolved in using materials having a dielectric constant on the order of15 or higher begins to degrade the performance of the non-reciprocaldevice in which they are used. It then follows that the efficiency ofthe non-reciprocal effects is limited in dielectric loaded devices bythe ratio 3 of velocities of propagation which, in turn, is limited bythe dielectric loss of the dielectric materials.

It is, therefore, an object of the present invention to provide anetficient, compact and simple waveguiding structure for non-reciprocalwave transmission having an inhomogeneous cross section withoutdielectric materials. In brief, the non-reciprocal wave transmissiondevice vof the present invention is a modified slow-wave-loaded'waveguiding structure with suitably mounted elements of agnetizedferrite. The slow-wave-loading is partial and is accomplished withconductive elements mounted in spaced-apart relation within thestructure to extend across one-half thereof to provide an inhomogeneouscross section and thereby a circularly polarized component of the radiofrequency magnetic field at the boundary of the loading elements. Theaction of the ferrite as mounted at the referenced boundary thenprovides non-reciprocal operation of the device.

Other objects and advantages of the invention will be readily apparentin the following description and claims considered together with theaccompanying drawings, in which:

FIG. 1 is a perspective view, partly in section, of a modified slow-wavecoaxial line having a non-reciprocal characteristic in accordance withthe present invention;

FIG. 2 is a characteristic curve showing the operation of the device ofFIG. 1; and

FIG. 3 is a modification of the device of FIG. 1.

Referring to the drawings in detail, FIG. 1 in particular, a section ofcoaxial line 11 comprises an inner conductor 12 and an outer conductor13. A plurality of semi-circular conductive disks 14 are similarly andradially mounted in parallel and spaced-apart relation on the innerconductor 12 to extend transversely with respect thereto. Such disks 14are electrically connected to the inner conductor 12 and insulated, asby suitable spacing, from the outer conductor 13. In this configuration,there is provided a modified slow-wave structure wherein energypropagates in a hybrid mode which is, in the general form, a combinationof the TB and TM modes.

The reason for the propagation of the referenced hybrid mode has beenset forth in the previous discussion and may be readily understood byconsidering that the velocity of propagation in the disk-loadedhalf-portion of the line is of one value and that of the unloadedhalf-portion is of another value. Since energy propagates with but onevalue of the velocity of propagation in such structure, the actualvelocity of propagation is one having a value that is intermediate tothe two referenced values. Thus,

the inhomogeneity introduced by the partial disk-loading of the innerconductor requires that the resulting mode have a longitudinal componentof the radio frequency magnetic field which is combination with thecircular component provides a circularly polarized radio frequencymagnetic field at or near the boundary between the loaded and unloadedregions of the line.

The theory of operation of the slow wave coaxial line having completedisks mounted on the inner or center conductor is well-known in the artand is generally applicable to the partially-loaded structure describedin the preceding paragraphs with minor modifications. Suchpartially-loaded coaxial line is reciprocal in operation in that wavespropagated in either direction are effected in the same manner.

Now to provide a non-reciprocal characteristic to the partially loadedcoaxial line two substantially thin elongated strips 16 and 17 offerrite material are mounted on the diametrical edges of the disks 14 toextend parallel to the inner conductor 12 with one on either side ofsuch conductor. Thus, the strips 16 and 17 are disposed at the boundarybetween the loaded and unloaded regions of the coaxial line structurewhere the circularly polarized radio frequency magnetic field of thehybrid mode of propagated energy is present. An adjustable staticmagnetic field, H, as indicated by arrows 21, is established 4 in aconventional manner to extend diametrically through coaxial line 11parallel to the broad faces of the strips 16 and 17.

With the ferrite strips 16 and 17 suitably magnetized, the electronspins within the ferrite and the sense of rotation of the circularlypolarized radio frequency magnetic field are properly related for onlyone direction of energy propagation and so provide a non-reciprocaldifferential phase shift and attenuation.

A 3 length of modified slow-wave coaxial line, as described in theforegoing, having a outside diameter and propagating energy at 3000mc./s. operates as indicated in FIG. 2. Such FIG. 2 is a plot of thedifferential phase shift in degrees obtained over a substantially lowincrement of applied static magnetic field of O to 400 gauss and aresulting curve 26 indicates a range of diiferential phase shift betweenzero and approximately 150 degrees. Similar operation of ahalf-slow-wave caxial line 1.3" in length has provided a differentialphase shift of +5 degrees with a loss of 0.3 db. with the foregoingstructures a VSWR of 1.1 or less over a 12% band width has been obtainedby providing matching transformer structure (not shown) at each end ofthe line.

The results described in the foregoing paragraph show the usefulness ofthe present invention as a differential phase shifter, which has manyapplications in the microwave art. In such device the ratio of thevelocities of propagation for the loaded and unloaded regions is of theorder of 10 to 1. From this it follows that to obtain the same ratio ina dielectrically-loaded waveguiding structure, a material having adielectric constant of the order of would be required. The lossesinherent in a material having such high dielectric constant would beprohibitive and are entirely avoided by the structure of the presentinvention. Also, by increasing the static magnetic field to a valueproducing gyro resonance in the ferrite strips 16 and 17, the device isoperable as an isolator with substantially high values of attenuationfor one direction of propagation and substantially none in the oppositedirection.

Referring now to FIG. 3 there is illustrated a second non-reciprocalcoaxial wave transmission line 31. In this embodiment a plurality ofconductive half disks 32 are radially mounted in spaced-apart andparallel relation in contact with the outer conductor 33 and extendtoward, but not in contact with the inner conductor 34. The resultingstructure also constitutes a slow-wave-loaded coaxial line and operatesin substantially the same manner as set forth for the structure ofFIG. 1. Thus, elongated ferrite strips 36 and 37 mounted on either sideof and parallel to the inner conductor 34 on the diametric edges of thehalf disks 32 provide a non-reciprocal transmission characteristic whensuitably magnetized. To provide the required magnetization, aconventional adjustable static magnetic field structure (not shown) ismounted externally of the line and establishes a magnetic field, H,through the ferrite strips 36 and 37 parallel to the broad faces thereofas indicated by arrows 39.

The operation of this latter embodiment of the invention is the same asset forth for the structure of FIG. 1. Thus, for substantially lowvalues of applied static magnetic field, difi'erential phase shift ofthe energy, propagating in one direction through the line, is obtained.Also, by increasing the value of the static magnetic field for gyroresonance in the ferrite, non-reciprocal attenuation results.

With respect to the foregoing non-reciprocal wave transmission device,it is to be realized that the 50% loading factor provided by thesemi-circular disks is not limiting. Other geometric configurationsproviding an inhomogeneous cross section are within the scope of theinvention set forth. Also, it is to be realized that the waveguidingstructure is not limited to circular coaxial lines as the sameprinciples are applicable with respect to other structures in which thedominant mode of propagation is the TEM mode such as rectangular coaxialline, strip line, and slab line.

Structure has, therefore, been described for providing non-reciprocalwave transmission with minimum losses and physical dimensions. While thesalient features of such structure have been described in detail withrespect to particular embodiments, it will be readily apparent thatnumerous modifications and changes may be made Within the spirit andscope of the invention and it is, therefore, not desired to limit theinvention to the exact details shown except insofar as they may be setforth in the following claims.

What is claimed is:

1. A non-reciprocal wave transmission device comprising: a coaxial typewaveguide supporting radio frequency energy in the TEM mode, saidWaveguide having an inner conductor and an outer conductor; slow-wavemeans disposed within said waveguide, said means including a pluralityof spaced-apart conductive half-discs mounted on one of said conductorsand spaced from the other conductor, said half-discs being similarlymounted in parallel relation to provide two regions in said waveguidehaving difierent velocities of propagation; a pair of elongated ferritestrips mounted on said half-discs parallel to said inner conductor alongthe boundary between said regions with one strip on either side of saidinner conductor; said waveguide being operative in a transverse staticmagnetic field paralleling the broad surfaces of said ferrite strips.

2. A device as claimed in claim 1, wherein said discs are connected tosaid inner conductor.

3. A device as claimed in claim 1, wherein said discs are connected tosaid outer conductor.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Seidel: Journal of Applied Physics, February 1957, pages218-226.

Morgenthaler et 211.: Proceedings of the IRE, November 1957, pages1S511552.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Robert L. Fogel etale 5 in the above numbered patfied that error appear tters Patentshould read as It is hereby certi n and that the said Le ent requiringcorrectio corrected below.

"where" insert there d hybrid line 53 Column 1, line 25, after "caxial"read line 39, for "hybrid" rea column 3, for "is" read in column 4, line18?, for coaxial line 19,, for "90+5" read 90-i 5 line 20 for "with"second occurrence, read With Signed and sealed this 22nd day of Januar(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of Patents

